1. Field of the Invention
This invention relates generally to devices and methods for shielding and deflecting missiles, mortars, and other projectiles launched from a ground launch location across a border toward a ground target location.
2. Background Art
Helium is much lighter-than-air. The difference is not as great as it is between water and air (a liter of water weighs about 1,000 grams, while a liter of air weighs about 1 gram), but it is significant. Helium weighs 0.1785 grams per liter. Nitrogen weighs 1.2506 grams per liter, and since nitrogen makes up about 80 percent of the air we breathe, 1.25 grams is a good approximation for the weight of a liter of air. There are about 28.2 liters in each one cubic foot. Thus, the lift of one cubic foot of helium in the air is about 28.2 grams.
One can determine the lifting capacity of the helium based upon the volume of the balloon. Assuming a lighter than air device (generally referred to herein as a “balloon”) is spherical, the volume may be determined from the radius of the balloon. The volume of a sphere is 4/3·π·r3, where r is the radius of the balloon. The radius of the sphere is half the diameter. Cube the radius (multiply it by itself twice: r3=r·r·r), multiply by π (approximately 3.14) and then multiply by 4/3. By measuring the balloon, in feet, the volume of the balloon is calculated in cubic feet. One cubic foot of helium will lift about 28.2 grams, so multiplying the volume of the balloon by 28.2 will give the lift force. Divide by 448—the number of grams in a pound—to determine the number of pounds it can lift.
Consider, for example, a 20-foot balloon has a radius of 10 feet. The calculation of the volume is 10 ft·10 ft·10 ft·3.14·4/3=4,186 cubic feet of volume. The calculation of lifting force (or buoyancy) is 4,186 cubic feet×28.2 grams/cubic feet=118,064 grams. This is converted into pounds of lifting force as 118,064 grams/448 grams per pound=263 pounds of lifting force. In another example, a 100-foot-diameter balloon can lift about 33,000 pounds! A 100 foot diameter balloon has a radius of 50 feet. 50·50·50·3.14·4/3=523,333.34 cubic feet of volume. 523,333.34 cubic feet×28.2 grams/cubic foot=14,758,000 grams/448 grams per pound=32,942 pounds of lifting force. Other lighter-than-air balloons, aerostats, and airships may come in different shapes for which the volume may be calculated or measured to provide the desired amount of lifting force. Unless otherwise indicated by the context, or unless otherwise specifically indicated, the term “balloon” may be used in this application to refer to any type of lighter-than-air devices whether gas filled balloons, hot air balloons, weather balloons, aerostats, blimps, dirigibles, or other lighter-than-air airships.